The present application claims priority to Japanese patent application of Kobayashi et al, filed Mar. 29, 2001, No. 2001-097327, and Japanese patent application of Kobayashi et al, filed Oct. 24, 2001, No 2001-326365, the entirety of each is hereby incorporated into the present application by this reference.
1. Field of the Invention
The present invention relates to an airbag for an airbag device and a method for manufacturing the airbag. The airbag device is mounted in the steering wheel of a vehicle or in the instrument panel in front of a passenger""s seat.
2. Description of the Related Art
In the prior art, the airbag of the airbag device is provided with a vent hole for discharging an inflating gas from the airbag. This airbag is inflated to a predetermined shape by the inflating gas fed from an inflator. At this point, the inflating gas is then discharged through the vent hole from the airbag so that its internal pressure does not exceed a predetermined value.
The time for the airbag to expand to the predetermined shape is longer if the vent hole is opened from the beginning of expansion of the airbag. Therefore, the vent hole is preferably formed when the internal pressure of the airbag reaches a constant value or higher. This vent hole is formed, for example, by forming a group of slits in the peripheral wall of the airbag. This slit group is formed by arranging a plurality of slits intermittently along one line. In this slit group, portions which are designed to rupture between slits are ruptured to form the vent hole for discharging the inflating gas (as referred to Unexamined Published Japanese Utility Model Application No. 9-134).
In the airbag of the prior art, however, the peripheral wall itself is made of a woven fabric of a synthetic resin. Specifically, the woven fabric was formed by weaving warp and weft threads of a synthetic resin of polyester or polyamide. Moreover, the airbag has to be compactly arranged when it is mounted as the airbag device on a vehicle. Therefore, the airbag is folded and housed in the airbag holding apparatus.
If the airbag is folded in the condition with the slits being formed in the peripheral wall of the airbag, moreover, the warps and wefts around the slits may become frayed or shifted out of position. Among airbags, therefore, during the formation of the vent holes there may occur differences in the effective area of the vent holes or in the timing by which the vent holes open.
An object of the present invention is to solve the aforementioned problems. Specifically, the object of the invention is to provide an airbag for forming a vent hole by a rupturing portion designed to rupture and, more particularly, an airbag which can stabilize the effective area and the opening timing of the vent hole, and a method for manufacturing the airbag.
The above-specified object is achieved by an airbag of the present invention. An airbag of the invention comprises a peripheral wall, a slit group, a portion designed to rupture and a vent hole. The peripheral wall is flexible and has portions which can be melted and can be solidified. The slit group is arranged in the peripheral wall and includes a plurality of slits arranged intermittently along one line. The portion designed to rupture is arranged between the slits of the slit group. The vent hole is formed along the line of the slit group by the rupture of the portion designed to rupture. The vent hole when opened discharges inflating gas. Moreover, the airbag of the invention further comprises portions formed in the peripheral wall at the peripheral edges of the slits, these portions solidifying after being melted.
Even when the airbag of the invention is folded, the melt-solidified portions prevent the fray and dislocation of the warps and wefts of the woven fabric at the peripheral edges of the slits. Therefore, the length and the rupture strength of the portion designed to rupture between the slits can be kept constant for every airbag. As a result, the opening timing at the time of forming the vent hole can be stabilized for every airbag. Moreover, the arranging position of the end portions of the slit group can be clearly specified for every airbag. Therefore, the effective area of the vent hole can also be stabilized for every airbag.
In the airbag of the invention, therefore, the effective area and the opening timing of the vent hole to be formed by the rupture of the portion designed to rupture can be stabilized for every airbag.
If the melt-solidified portions are formed all over the slit peripheral edges, moreover, the shape holdability of the slits can be retained. Therefore, the inflating gas to leak from the narrow clearances within the slits in the course of expansion of the airbag can be controlled to be constant. As a result, the time period from the start to the end of expansion of the airbag can also be stabilized for every airbag.
In the airbag of the invention, for example, the peripheral wall includes a main body cloth and an applied cloth having the slit group arranged therein. The main body cloth has an arranging hole opened for exposing the slit group to a portion for forming the vent hole and in a smaller shape than the outer shape of the applied cloth. The applied cloth is integrated with the main body cloth, the slit group being visible from the arranging hole, by sewing its peripheral edge to the peripheral edge of the arranging hole.
In this airbag, tensile forces are countered by the sewn portions of the peripheral edges of the applied cloth, even if they act on the peripheral wall at the peripheral edges of the applied cloth at the expansion time. Before the internal pressure of the airbag reaches a predetermined value (that is, before the vent hole is properly opened), therefore, it is possible to prevent a premature rupture at the portion designed to rupture on the applied cloth. As a result, the timing for forming the vent hole can be more stabilized for every airbag.
In the airbag of the invention, moreover, the peripheral wall includes a main body cloth and an applied cloth. The applied cloth is arranged over the main body cloth at a portion for forming the vent hole, by sewing its peripheral edge. The slits are individually cut through the applied cloth and the main body cloth. Moreover, the melt-solidified portion is formed by melting and fusing the applied cloth and the main body cloth to each other and then solidifying.
In this airbag, too, around the portion for forming the vent hole, there is formed the sewn portion in which the peripheral edge of the applied cloth is sewn to the main body cloth. Like the aforementioned airbag, therefore, even if pulling tensile forces act on the peripheral wall at the peripheral edges of the applied cloth when the airbag expands, the tensile forces are countered by the sewn portions of the peripheral edges of the applied cloth. Before the internal pressure of the airbag reaches a predetermined value, therefore, it is possible to prevent a premature rupture at the portion designed to rupture. It is also possible to stabilize the opening timing of forming the vent hole. Moreover, at the melt-solidified portions around the slits the overlaid applied cloth and main body cloth have been melted, fused thickly, and solidified together. In other words, the melt-solidified portions are formed to retain the highest shape holdability. Therefore, even if strong tensile forces act on the sewn portions at the peripheral edges of the applied cloth, i.e. the vent hole forming portion, in the course of expansion of the airbag, the portion designed to rupture does not rupture easily. As a result, premature rupture before the internal pressure of the airbag reaches the predetermined value is prevented as much as possible. It is also possible to further stabilize the opening timing at the vent hole forming time for every airbag.
Moreover, the line of the slit group may be bent and arranged in the peripheral wall so that the vent hole may be opened with a flap portion. The hinge line of the flap portion is arranged at the straight line joining two ends of the slit group. Moreover, the flap portion is arranged at a portion which is surrounded by the hinge line and the slit group, so that the vent hole is formed when the flap portion is opened bending at the hinge line.
In this airbag, the area of the vent hole can be more uniform among all airbags than the airbag where the vent hole is formed by arranging the slits merely in a straight line. Here, the vent hole formed by the slit group having the slits arranged in one straight line opens with its peripheral edges opened perpendicularly. If the opening width fluctuates, moreover, the effective area will easily change.
Where there is to be a bend in the arrangement of the slit group, it is desired that a slit originates at the bending point and continues outward in the shape of radiation focusing on the bending point. With this construction, when the vent hole is formed, the tearing forces passing through the bending point are not easily propagated to the peripheral wall around the bending point. Therefore, it is possible to prevent the peripheral wall from rupturing more than necessary.
Moreover, The slits of the slit group may be so individually arranged in an H-shape, as viewed in a top plan, that the two flap portions may be opened to open one vent hole when the internal pressure of the airbag rises. The opening shape of the vent hole is a rectangle. With this slit group, the following actions and effects can be acquired, as compared with the case in which the individual slits are arranged in a C-letter shape in a top plan view so as to form the same opening shape with one flap portion.
Specifically, the length of the side edges of the flap portion adjoining the hinge lines can be made shorter for the vent hole with two flap portions than that with one flap portion. Therefore, it is possible to reduce the tearing inertial forces along the side edges at the rupture time (at the door opening time). It is also possible to prevent such tearing from elongating the rupture at the two ends of each hinge line. As a result, it is possible to improve the uniformity of the effective area.
Thus, where a plurality of slits are arranged in a straight line, it is desired that trifurcated slits opened in a T-shape or an inverted T-shape are arranged at the two ends of the series of slits and such that ends of the parts of the T-shape or inverted T-shape toward the ends of the above straight line are directed to each other.
In this construction, the two flap portions are opened when the portion designed to rupture between the slits rupture. The hinge lines of the individual flap portions are the straight lines joining the ends of the transverse openings in the trifurcated slits. The leading edges of the individual flap portions are arranged on the side of the longitudinal openings of the two trifurcated slits. Merely by rupturing the portion designed to rupture arranged in the straight line portion, therefore, the vent hole having a wide effective area and a rectangular shape can be easily formed.
Moreover, it is desired that the straight line portion is constructed with at least one straight slit arranged between the two trifurcated slits. With this construction, the effective area of the rectangular vent hole can be easily adjusted if the number or length of the straight slits is changed.
In case the slit group includes a portion having a plurality of slits arranged straight, moreover, it is desired that this portion is arranged such that the directions of the individual slits are those of tensile forces which act on this portion for forming the vent hole in the peripheral wall when the inflating gas flows in prior to the formation of the vent hole. That is, the arrangement of slits is parallel to the tensile forces at the initial stage of the expansion of the airbag.
With this construction, the tensile forces to act in the directions to tear the portion designed to rupture in the course of expansion of the airbag are not applied to the vent hole forming portions. As a result, the portion designed to rupture at the straight line of the slit group is prevented from prematurely rupturing at the initial stage of expansion of the airbag before the internal pressure of the airbag reaches a predetermined value.
In case the portion of the peripheral wall in which the slit group is arranged is made of a woven fabric which is formed by weaving molten solidifiable warps and wefts and in case the slit group has a straight line portion in which a plurality of slits are arranged straight, moreover, it is desired that the array directions of the individual slits in the straight line portion are bias directions to intersect both the directions of the warps and the wefts.
With this construction, the portion designed to rupture of the straight line portion can be prevented from unnecessarily rupturing at the initial stage of expansion of the airbag. Specifically, when the portion designed to rupture of the straight line portion ruptures, the tensile forces to act on the portion designed to rupture acts generally perpendicular to the direction of the individual slits of the straight line portion. However, the directions of the tensile forces are biased to both the directions of the warps and wefts constructing the portions of the peripheral wall having the slit group. In other words, the directions of the tensile forces are those in which the woven fabric composed of the warps and wefts are easily extended. Therefore, the woven fabric of the warps and wefts can be stretched to reduce the tensile forces acting on the straight line slit portion. As a result, the portion designed to rupture between the slits of the straight line portion can be prevented from unnecessarily rupturing at the initial stage of expansion of the airbag before the internal pressure of the airbag reaches a predetermined value.
In case the angle of intersection between the direction of the individual slits of the straight line portion and the direction of the warps or the wefts is 45 degrees, both the direction of the straight line portion and the direction perpendicular to the straight line (i.e., the acting directions for the tensile forces to rupture the portion designed to rupture) are right bias directions or positive bias directions (i.e., the directions to intersect the directions of the warps and wefts individually at 45 degrees), in which the woven fabric is most extendible. In the case of the intersection angle of 45 degrees, therefore, the tensile forces to act on the portion designed to rupture can be most reduced to prevent the unnecessary rupture of the portions designed to rupture at the initial stage of expansion of the airbag. Even if the array directions of the straight line portion are less than 45 degrees for the intersection angle with the warps or wefts of 10 degrees or more, however, the woven fabric of the warps and wefts is extendible. Therefore, the tensile forces to act on the straight line portion in directions substantially perpendicular to the straight line portion can be reduced to some extent. As a result, the intersection angle between the straight line portion and the warps or wefts may be 10 to 45 degrees.
In case the peripheral wall is formed for a steering wheel by sewing the outer peripheral edges of a circular open side base cloth and a circular passenger""s side base cloth to each other and in case the open side base cloth has a gas inlet port opened at its center for introducing the inflating gas and portions for forming the two vent holes, moreover, it is desired that the straight line portion is constructed in the following manner. Specifically, the straight line portion is arranged at each of two transversely symmetric portions on the longitudinal center axis extending through the gas inlet port in the open side base cloth. Moreover, the two straight line portions are individually arranged such that the individual slits are in a radial direction from the gas inlet port on the open side base cloth and tilted up to 45 degrees from the center axis on the front side of the gas inlet port.
In this airbag, the vent holes are located on the front side away from the back side of the ring portion which is gripped by the driver. As a result, the inflating gas to be discharged from the vent holes can be prevented from touching the hands of the driver directly. Of course, the directions on the open side base cloth radiating from the gas inlet port are oriented along the directions for the tensile forces to act on the open side base cloth at the time of extending and expanding the airbag. If the directions of the straight line portion are arranged in these radial directions of the open side base cloth, therefore, the tensile forces to act in the directions to tear the portion designed to rupture act as little as possible on the vent hole forming portion in the course of expansion of the airbag.
In a method for manufacturing an airbag of the invention, the slits are individually formed while the peripheral edges of the slits are melted at the time of forming the individual slits. After the slits are formed, the molten peripheral edges are solidified to form the melt-solidified portions at the peripheral edges of the individual slits in the peripheral wall thereby to form the slit group. In this method, it is possible to form the slits and melt the slit peripheral edges simultaneously by using a heated cutter or a laser cutter. After this, the slit peripheral edges are air-cooled and solidified, and the melt-solidified portions are formed. Therefore, the slits and the melt-solidified portions can be easily formed with few manufacturing steps, resulting in lower cost of the airbag.
Moreover, the end portions of individual slits which are close to the portion designed to rupture may be made wider than the portion of the slits away from the ends. With this construction, the end portions of the individual slits adjacent to the portion designed to rupture are widened. Therefore, the visual observation of the portion designed to rupture is facilitated. As a result, the length of the portion designed to rupture can be efficiently confirmed.